a) Field of the Invention
This invention relates to a method of controlling a generator output of a generator mounted on a so-called hybrid vehicle, namely, an electric power generation control method in a hybrid vehicle.
b) Description of the Related Art
An electric vehicle is a vehicle using a motor as a driving source and a so-called hybrid vehicle is known as one type of electric vehicle. The hybrid vehicle is an electric vehicle equipped with an engine in addition to a motor. Some hybrid vehicles, wherein a generator is driven by an engine and a power generation output of the generator can be used for driving the motor, are called series hybrid vehicles (SHVs).
FIG. 6 is a block diagram showing the configuration of one example of an SHV. The vehicle shown in the figure uses an AC induction motor 10 as a driving source. An output of the motor 10 is transmitted via a transaxle 12, etc., to driving wheels 14 for use as a driving force of the vehicle.
The vehicle is equipped with a chargeable and dischargeable battery 16 such as a lead battery. A discharge output of the battery 16 is converted into three-phase AC power by an inverter 18 made up of a plurality switching elements, and the power is supplied to the motor 10. At this time, the inverter 18 is controlled by an ECU (electronic control unit) 20. That is, the ECU 20 calculates reference torque indicating torque to be output from the motor, in response to the driver of the vehicle who presses down on an accelerator or brake pedal, and controls the inverter 18 in response to the found reference torque, thereby controlling the power supplied to the motor 10, so that torque corresponding to the reference torque is output from the motor 10.
In addition to the battery 16, a generator 22 is mounted as means for supplying power via the inverter 18 to the motor 10. The generator 22 is connected via a speed increasing unit 24 to an engine 26. When the ECU 20 operates the engine 26, an output of the engine 26 is transmitted via the speed increasing unit 24 to the generator 22. At this time, when a field current is supplied to the generator 22 under the control of the ECU 20, a power generation output is provided from the generator 22. The generator 22 shown in FIG. 6 is a three-phase AC generator and an output of the generator 22 is rectified by a rectifier 28 for supply to the inverter 18 and the battery 16. Therefore, the motor 10 can be driven by an output of either the battery 16 or the generator 22 and the battery 16 can be charged not only by regeneration of the motor 10, but also by an output of the generator 22. The speed increasing unit 24 is a mechanism for raising the number of revolutions of the engine 26 to a value corresponding to the generator 22.
To run the engine 26, the ECU 20 controls run conditions of the engine 26 so that the engine 26 rotates in the region having the best fuel economy, as shown in FIG. 7, for example. The run conditions to be controlled include fuel injection, spark advance, throttle angle, etc., for example. The best fuel economy region is in the range of 1200 rpm to 2800 rpm with respect to a four-cylinder engine, for example. Normally, the emission contents of exhaust gases from the engine 26 are improved in this region.
To provide required power from the generator 22 while such highly efficient running of the engine 26 is being performed, the ECU 20 controls the field current of the generator 22, for example, whereby a generator output from the generator 22 can be controlled to the target value while the engine 26 is driven in the best fuel economy region. Such control is disclosed in Japanese Patent Laid-Open No. Sho 51-39813, etc., for example.
However, if generator output of a generator 22 is controlled by controlling the run conditions of an engine 26 and the field current of the generator 22, the number of revolutions of the engine 26 may not necessarily be a value falling within the best fuel economy region. That is, the individual characteristics of engines and generators vary from one to another in mass production and the difference between the individual engines or the difference between the individual generators changes over time. When such variations or changes with time occur, even if the generator output according to the target value is provided from the generator 22 by controlling the field current, the number of revolutions of the engine 26 will still vary.
When the field current of the generator 22 is small, the number of revolutions of the engine 26 increases; when it is large, the number of revolutions of the engine 26 decreases. Further, when the number of revolutions of the engine 26 changes, not only an output of the engine 26, but also generator output of the generator 22 changes accordingly. However, the characteristic varies depending on whether the load of the engine 26 is high or low (for example, whether the throttle angle is large or small). That is, for example, the engine output characteristic of a gasoline engine at equal throttle when load is high follows a trend to move up to the right with respect to the number of revolutions of the engine as shown in FIG. 8A; the engine output characteristic at equal throttle when load is low follows a trend to move down to the right with respect to the number of revolutions of the engine as shown in FIG. 8B. Therefore, even if an attempt is made to control a generator output of a generator 22 by changing the field current of the generator 22 as in the related art, the generator output change trend when the engine load is high differs from that when the engine load is low.